A flexible substrate (i.e. flexible printed circuit) has a fundamental structure composed of a conductor and a heat-resisting polymer film. A flexible substrate in which the conductor is disposed only on one side of the heat-resisting polymer film is called “single-sided flexible substrate”. In contrast, a flexible substrate in which conductors are disposed on both sides of the heat-resisting polymer film is called “both-sided flexible substrate”.
A copper clad laminate (CCL) is generally used to produce the single-sided flexible substrate. There are two kinds of copper clad laminates, wherein one is a three-layer CCL in which a copper foil is disposed on the heat-resisting polymer film via an adhesive, and the other is a two-layer CCL in which a copper foil is disposed on the heat-resisting polymer film without an adhesive layer. Such two-layer or three-layer CCLs are respectively produced by performance of a laminating process, a casting process or a sputtering/plating process. A subtractive process is performed by using the two-layer or three-layer CCLs, and thereby the single-sided flexible substrate is consequently obtained.
A laminating process of the single-sided or both-sided flexible substrates by using a film and an insulating resin layer leads to a multilayer flexible substrate. A through-hole conductor (i.e. via) is formed in the multilayer flexible substrate by coating an inner wall of a through-hole with a metal. Therefore, wiring patterns of respective layers in the multilayer flexible substrate are electrically connected to each other.
Such flexible substrate or multilayer flexible substrate can be effectively used for a spatially narrow mounting-area due to flexibility thereof. For example, the flexible substrate or multilayer flexible substrate is mounted not only for a small space around a compact liquid crystal provided for in a camera, a cell-phone or a portable PC, but also a small space around PC peripheral equipment such as a printer or an HDD. Recently, further high density and performance of a semiconductor has been required as an electronics device is becoming more compact, lighter and thinner. Therefore, a flexible substrate having a semiconductor or a passive element is also required to be thinner and of high-density. For example, not only an increase of an output terminal number, but also a fine pad pitch of a driver IC is required with progress in terms of colorization and high definition of a liquid crystal display.
With respect to the foregoing, related documents are as follows:                Japanese Patent Kokai Publication No. 11-157002 (see 3 page thereof);        Japanese Patent Kokai Publication No. 2004-31588 (see 2 page thereof);        Japanese Patent Kokai Publication No. 4-107896 (see 1 and 2 pages thereof);        Japanese Patent Kokai Publication No. 2-180679 (see 1 page thereof);        Japanese Patent Kokai Publication No. 10-256700 (see 2 and 3 pages thereof);        Japanese Patent Kokai Publication No. 2000-77800 (see 1 page thereof);        Japanese Patent Kokai Publication No. 2003-224366.        
A conventional flexible substrate and a conventional process for producing the same were fraught with problems as shown in following matters (I)-(VII):
(I) Miniaturization of a wiring pattern is important from a standpoint of producing a thinner and high-density flexible substrate. There is a limitation of fine wiring of a wiring pattern with regard to a subtractive process (i.e. chemical etching), because a thickness of a copper foil used in the flexible substrate is from 18 to 35 μm. That is to say, it is difficult for the subtractive process to produce an at most 75 μm line width of a wiring pattern by using copper foil having a thickness of 18-35 μm. As a result, a thinner copper foil is needed for attaining a further miniaturization of the wiring pattern.
(II) In a case where the subtractive process such as a chemical etching is used to produce a wiring pattern, there is a possibility that an etchant is left behind between wiring patterns, which in turn adversely affects a reliability of electrical insulation. The subtractive process provides such a construction that a wiring pattern protrudes from a surface of a substrate. This protrusion of the wiring pattern results in a decrease of a surface flatness in the substrate. Thus, there is a possibility that a bump provided in a semiconductor chip is somewhat difficult to be mounted on the wiring pattern. Also, there is a possibility that a mounted bump is moved between wiring patterns, and therefore a short-circuit occurs. Also, the protrusion of the wiring pattern itself may cause to interrupt a plastic molding process that is performed afterward.
(III) A through-hole conductor is generally used to connect wirings of respective layers to each other. This indicates that an increase in a number of the layers will lead to an increase in a number of through-hole conductors, which in turn will result in less sufficient space for the wirings. Therefore, it is a general method to laminate single-sided flexible substrates provided with through-hole conductors or both-sided flexible substrates provided with through-hole conductors. In this case, the through-holes are filled with a metal paste. Such metal paste inevitably contains a liquid resin or solvent in terms of efficient filling and printing, and thus a resulting circuit has a higher resistance than that of a conventional circuit prepared by performing a copper plating process. With a decrease in a diameter of the through-hole, the through-holes become increasingly harder to be filled with the metal paste. Thus, a viscosity and fluidity of the metal paste are required to be adjusted by adding a large amount of solvent to the metal paste. This results in an evaporation of the solvent contained in the metal paste, which in turn leads to a formation of gas cavities. As a result, resistance of the through-hole itself increases due to the gas cavities.
(IV) In a case where a through-hole conductor is formed, an adhesive layer and a film are respectively perforated by performance of laser machining. The adhesive layer is easy to be machined with a laser, while on the other hand, a thick film used in a conventional flexible substrate is difficult to be machined with a laser. Concretely, a hole obtained by laser-machining a conventional organic film is not circular in shape due to heat during this laser machining process, which in turn leads to a burr. Also, metal paste is difficult to be poured into the hole due to the fact that a diameter of an incident laser is smaller than that of an outgoing laser.
(V) A fine wiring pattern or through-hole conductor as well as a thin circuit component is important from a standpoint of producing a thinner and high-density flexible substrate. Passive elements such as an inductor, a condenser or a resistor are generally mounted on a surface of the substrate in such a manner that they protrude from the surface of the substrate. This will cause such a problem that the substrate becomes thick as a whole.
(VI) In a conventional flexible substrate, a passive or an active element is formed on an exposed surface of the substrate, so that the passive or the active element is not included within the flexible substrate. Thus, in a case where a multilayer substrate is produced by use of such flexible substrate, a multilayering process is performed against the passive or the active element formed on the exposed surface, and consequently the passive or the active element is formed between respective layers as well as between wiring patterns. Therefore, this will cause such a problem that a region for the wiring patterns becomes smaller.
A flexible substrate is needed to be folded in a smaller space. Thus, a better flexing life (or sliding flexibility) of the flexible substrate is required. Therefore, a sufficient flexing life of a two-layer CCL is required and thereby a high adhesion strength between a polyimide film and a copper foil is also required. Further, in a case of a three-layer CCL, in addition to the above adhesion strength, a high adhesion strength among a polyimide film and a copper foil, and an adhesive composition, is required.
Furthermore, a conventional wiring pattern formed by an etching process is exposed to its surroundings on surfaces of a flexible substrate. This will cause a microcrack in wiring patterns when the flexible substrate is folded, which will be far from satisfying in terms of the flexing life.
Considering the challenges or problems as described with respect to the above (I) to (VI), an object of the present invention is to provide a reliable, high density and thin flexible substrate that is better in terms of flexing life. A further object of the present invention is to provide a process for producing such flexible substrate.